An essential part of any radio transceiver is the arrangement for switching between transmit and receive modes. With many transceivers, including most amateur and citizens' band radios, the switching into transmit mode is accomplished by the user pressing a "transmit" button. Such an arrangement, called "push-to-talk", gives the user an unequivocal indication of whether the transceiver is in transmit mode or receive mode. If the button is pressed, the transceiver is in transmit mode, and upon the release of the button, the transceiver goes into receive mode.
Another technique for controlling the switching of a transceiver between receive and transmit modes, instead of using a push-to-talk button, is the use of voice actuation. This technique, called "VOX", used circuitry to monitor the signal level from the microphone. When the signal level exceeds a predetermined threshold, the transceiver switches to transmit mode. Later, the signal level will be found to have dropped below a threshold (which may or may not be the same as the previously mentioned threshold), and when the below-threshold condition persists for longer than a predetermined interval, the transceiver switches back to receive mode. The usual (or at least the desired) result when VOX is employed is that the transceiver switches to transmit mode when the user speaks, and returns to receive mode after the user has finished speaking.
While VOX operation has the advantage that the use can be handsfree, that is, that one need not have one's hands free for pressing the PTT button, it can also have disadvantages.
One disadvantage relates to the user's potential inability to recognize an "open mike" condition. For example, the transmit threshold appropriate for one situation, such as a quiet room, may be inappropriate for another, such as a noisy room. Where the ambient noise level is high, the ambient noise by itself, reaching the microphone, may reach the transmit threshold and cause the transceiver to enter transmit mode. This will result in unwanted transmission of the modulated carrier, which at the very least has the drawback of needlessly consuming power from the power supply, typically a battery. The unwanted transmission has the further drawback of filling the channel and thus interfering with other transceivers that would transmit on the channel. Finally, there is the problem that information not intended for transmission may be transmitted anyway.
Where the communications channel is full duplex (e.g. one channel is used for transmit and another for receive) the abovementioned problems of unnecessary power consumption, domination of the transmission channel, and unwanted transmission of information may arise. With many transceivers, however, the communications channel is not full duplex, that is, a single channel is used both for transmission and reception. With such transceivers, each of the transceivers transmits on that channel and each of the transceivers (when not in transmit mode) monitors that channel for receiving information. If and when a particular VOX-type transceiver of this type gets "stuck" in transmit mode due to a continuing signal in excess of the transmit threshold, there is the further problem that for the duration of the "stuck" transmit interval, the user of that transceiver will be unable to hear anything from other transceivers because the transceiver hearing signals from other transceivers requires that the transceiver not be in transmit mode.
Another drawback of VOX operation is that the user may not be aware of the battery having run down, other than by noting the prolonged absence of an audible signal in the earphone.
Yet another drawback of VOX transmission switching is evident, as discussed below, in the case of certain transceivers having a combined microphone and earphone. Such a transceiver picks up the user's speech by conduction through the earphone, and offers the feature that there is no requirement of a microphone positioned at or near the mouth. Another feature of such a microphone arrangement is that it may be less susceptible to ambient noise than a microphone of the type that receives air-transmitted sound energy from a position near the mouth. A VOX-equipped transceiver with combined microphone and earphone offers a hands-free, unobtrusive communications capability.
But in a transceiver having combined microphone and earphone, design consideration must be given to the problems that might arise due to close proximity of the microphone and earphone. It would be undesirable, for example, if received signals provided to the user through the earphone were picked up by the microphone, thus triggering a change to transmit mode. One way to avoid that difficulty is to provide a "squelch" circuit on the receiver, so that only received signals exceeding a set threshold are passed to the earphone. Signals that are fainter than the threshold are "squelched", i.e. are not provided to the audio amplifier that drives the earphone. The transceiver thus has two modes, often termed "quiet mode" (during which the receiver is active but is not providing a signal to the earphone) and "receive mode" (during which the squelch threshold has been exceeded and a signal is being provided to the earphone). The transceiver is configured so that the VOX circuit (which monitors the signal level at the microphone) is only enabled in quiet mode. Stated in the converse, the VOX circuit is disabled whenever the received signal exceeds the squelch threshold.
In a transceiver of the type just described, the reliance on VOX actuation thus has the severe drawback that there can be "transmit starvation". During the entire time that a strong signal (in excess of the squelch threshold) is being received, the user will not be able to cause the transceiver to switch to transmit mode by talking. This occurs because the VOX circuit is disabled when a received signal exceeds the squelch threshold.
The drawback of "transmit starvation" is exacerbated by the problem that the user may not be aware that talking is not accomplishing the switch to transmit mode. As mentioned above, when a PTT button is pushed, there is an unambiguous indication that the transceiver is in transmit mode. But in VOX mode, there will generally be no indication of a successful voice-activated switch to transmit mode, and thus no clue that an attempt to switch to transmit mode was unsuccessful.
The combined result of the abovedescribed drawbacks is that communication between two users can be frustrating if both are using VOX transceivers with combined microphone and earphone. One user may happen to cause an open mike condition, due to (say) allowing headgear to rub against the microphone/earphone and continuously exceeding the VOX threshold. The other user will be unable to transmit because the received signal, continuously exceeding the squelch threshold, will disable the VOX circuit for that user. In typical hands-free applications, such as skiing, bicycling, and the like, the transceiver is in a pocket or pack, so that even if there are indicator lights showing the modes (receive, quiet, and transmit) the users will not be able to see them. Thus the first user will be unaware that she is causing the open mike condition, and the second user will be unaware that her spoken words are not being received by the first user.
It is an object of the present invention to provide an improved transceiver in which users are unlikely to unknowingly cause "open mike" conditions. It is a further object of the invention to provide an improved transceiver in which a user will become immediately aware of the condition if unable to transmit. It is yet another object of the invention to provide an improved transceiver in which the user will more promptly learn of loss of battery power.